[rantanplan]

I've read about their reasons, I just didn't want to open a discussion about it :D

Deciding to implement constraints on the application/business layer is something that github can probably do, in the same vein that Facebook can create their own PHP compiler.

I just think that you can't use these examples as effective arguments on whether someone should use FKs or not. I guess whatever brute-force solutions these companies would apply, on any given problem, would probably work one way or the other.